Micro-objects, such as biological cells, can be processed in microfluidic devices. For example, droplets containing micro-objects or reagents can be moved around and merged within a microfluidic device. Embodiments of the present invention are directed to improvements in microfluidic devices that facilitate carrying out complex chemical and biological reactions. Droplets can be moved and merged on or within a microfluidic device by changing an effective wetting property of an electrowetting surface in the microfluidic device. Such movements can facilitate, for example, workflows in which cells are processed to assess various cellular properties. Microfluidic devices having an electrowetting configuration typically include a hard cover, which can complicate the introduction of droplets onto and their removal from the electrowetting surface. Accordingly, it is desirable to have microfluidic devices that have a more accessible electrowetting surface.